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## Motivation A lot of folks have expressed interest in being able to capture the current `tracing` span context when an error type is _constructed_, so it can eventually be displayed when the error is handled. Since span entry is stackful, the error may not be formatted or otherwise handled within a span context that is in any way related to the one where the error occurred — and this is only made worse when "channels" are involved. ## Solution This branch introduces a new crate, `tracing-error`, which will contain integrations and types for enriching errors with `tracing` span contexts. Currently, this crate contains the following: - A `SpanTrace` type, which captures the current span and allows it to be inspected later. - A `Layer` implementation which may be added to a `Subscriber` to enable capturing traces for errors. This layer will format fields and insert them into span extensions, **if no other layer has already added such an extension** using the same formatter. This reuses the formatting traits from `tracing-subscriber`'s `fmt` module. ## Notes Note that we have not actually added any new error types or error wrapper types. I would like to provide these in the future, so that span capturing can be seamlessly integrated into the rest of the Rust error handling ecosystem. However, this will take some additional experimentation. For now, we want to release the `SpanTrace` type so that other libraries and applications may use _it_ as an integration point. We'll need to add more documentation and examples for these APIs, and can do that in follow-up branches. Closes #464 Signed-off-by: Eliza Weisman <eliza@buoyant.io> Co-authored-by: Jane Lusby <jlusby@yaah.dev>